The platelet-derived growth factor (PDGF) stimulates BALB/c-3T3 cells to rapidly and specifically synthesize certain proteins which may be required for DNA synthesis. At least one of these proteins (pII; Mr,35,000) is constitutively synthesized by spontaneously transformed BALB/c-3T3 (ST2-3T3) cells which do not require PDGF for growth. Antisera against a major excreted protein family (MEP) of retrovirus-transformed cells quantitatively precipitated cellular pII. PDGFstimulated pII has the same molecular weight, a similar charge and similar antigenic determinants as authentic MEP isolated from ST2-3T3 or retrovirus-transformed cells. MEP represented about 2% of the nonnuclear proteins synthesized by ST2-323 cells and 0.3 to 0.6% of the proteins synthesized by PDGF-treated BALB/c-3T3 cells, a three-\to sixfold increase over the background. In BALB/c-3T3 cells, less PDGF was required for pII (MEP) synthesis than for DNA synthesis. PDGF induced a selective increase in pII (MEP) within 40 minutes. Such preferential synthesis was inhibited by brief treatment with actinomycin D, suggesting a requirement for newly formed RNA. The constitutive synthesis of pII (MEP) by ST2-3T3 cells was not inhibited by actinomycin D. Five spontaneously or chemical carcinogen-transformed tumorigenic BALB/c-3T3 cell lines were studied; they neither required PDGF for growth nor responded to it. These cell lines became arrested at confluence with a G1 DNA content. Each of these independently isolated lines synthesized pII (MEP) constitutively. Thus, the constitutive synthesis of pII (MEP) is associated with cellular transformation. PDGF-modulated pII (MEP) synthesis may be required, but is not sufficient, for DNA synthesis.